Secondary-emission electron discharge device



Dec. 16, 1952 D. A. JENNY 2,622,218

SECONDARY-EMISSION ELECTRON DISCHARGE DEVICE Filed Jan. 51, 1950ATTORNEY Patented Dec. 16, 1952 SECONDARY-EMISSION ELECTRON DISCHARGEDEVICE Dietrich A. Jenny, Princeton, N. J assignor to Radio Corporationof America, a corporation of Delaware Application January 31, 1950,Serial No. 141,385

Claims. 1

This invention relates to improved electronmultipliers and to improvedmethods of and apparatus for prolonging the useful life of thesecondary-electron emissive electrodes or dynodes of such devices.

Despite the many advantages of electron multiplier tubes such as verygreat gain and wide band operation, they have not come into widespreaduse as power devices because the active secondary emissive coatings onthe dynodes are rapidly exhausted under intense bombardment.

Various explanations have been oifered for the relatively short life ofthe dynode coating material and various practices have been followed forprolonging it. According to one View, the secondary emitter coating ispoisoned by evaporation products which reach it over straightline pathsfrom the cathode. One pratcice based on this view is to block thesepaths with shields which can be circumvented by electrons if they areappropriately deflected, and another is to make the cathode coatings ofspecial compositions and/or of limited areas to limit their rate ofevaporation to the secondary emitter. However, the use of shieldsreduces the frequency range of all such devices without avoiding coatingexhaustion in those used for power purposes.

According to another view, the coating decomposes under electronbombardment and one or more of its components evaporates. A prior artpractice based on this View is to use special secondary-emittercoatings, such as denser coatings, which take longer to be decomposed.In support of the practice of using denser coatings, it has beensupposed that such a coating will take longer to become exhaustedbecause it contains more useful active material (inasmuch as morematerial can be crowded into the coating without increasing its size,and, in addition, the coating can be made thicker for a required valueof front-to-back conductivity).

However, this practice has not been successful in providing a secondaryemitter having a reliably long useful life under high current densityconditions. The reason Why it has not been successful will be apparentfrom an analysis of secondary emission which is presented herein.

Accordingly, it is an object of the present invention to deviseimprovements in discharge devices using secondary electron emission inwhich the secondary emitter coatings can be depended upon to have longuseful lives even under high current density conditions.

It is a further object of the present invention to devise a process forproducing improved discharge devices of thekind set forth above.

It is a further object of the present invention to provide improvedmethods for operating discharge devices of the character indicated.

In general, according to the present invention these objects areattained by providing means for continuously reconstituting thesecondary emissive coating during operation so that it will not becomeexhausted but will remain useful as long as any coating remains. Thismeans includes a source for supplying an active gas which will combinewith a free metal component of the coating as rapidly as it loses gasmolecules due to electron bombardment.

In the drawing:

Fig. 1 is a longitudinal section through a secondary emission dischargedevice embodying the present invention;

Fig. 2 is a top view of the device of Fig. 1 with its envelope coverremoved;

Fig. 3 is an enlargement of a portion of Fig. 1 to illustrate asecondary emissive coating; and

Fig. 4 is a fragmentary sectional view of a modification of theembodiment of Fig. 1.

The five-stage electron multiplier shown in Fig. 1 comprises anevacuated metal envelope ID consisting of a bottom 52 and a hollow cover14 of the bathtub" type. Within the envelope the tube elements arecombined in a rigid assembly I5 which is bolted to the bottom B2. Inthis assembly the secondary emitter electrodes are stacked one above theother and separated by mica shims to provide the multiplier stages withboth D. C. isolation from each other and low impedance R. F. by-passesto ground. As is most apparent from Fig. 2, the assembly I? isrectangular in shape and is centrally positioned on the envelope bottoml2. At the bottom of the assembly is a base plate I8. This plate and amica shim l8 by which it is insulated from the envelope bottom l2 havealigned central'openings 20, 22 through which an anode support rod [9may extend in a manner to be described below. The right half of the baseplate It (as shown in the drawing) is thicker than its ieft half. Thiscauses the respective secondary emitter electrodes (or dynodes) whichare stacked on the opposite ends of the plate It to be at staggeredlevels so that a zig-zag flow of electrons will occur, during operation,as represented by the dot-dash line 23.

The first, third and fifth dynodes (24, 26, 28 respectively) are stackedon the left half of base plate l6, each being insulated from itsadjacent elements by two mica shims 39. Correspondingly, the second andfourth dynodes 32, 34

and a number of shims 38 are stacked on the a capsule 62, is formed ofsilver.

a right half of plate it. The dynodes are held tightly together betweenthe envelope bottom 12 and a top plate 36 by a number of bolts 38. Thebolts 38 are insulated from the dynodes and the top and base plates bydielectric sleeves, not shown, such as sleeves of ceramic material.

A recess 48 is formed on the under side of the top plate 35 across itscentral portion to provide space for an indirectly heated cathode t2 anda control grid 43. Cathode it comprises a nickel tube 44 supported atits ends by perforated mica covers (55 in Fig. 2). These covers areattached to the sides of the top plate 35 over the ends of its recessit. Two heater leads, '55, extend into the hollow nickel tube 6 3 fromits opposite ends to a heater winding, not shown, which is carriedbewteen them in the central portion of the nickel tube. These leads andthis heater should be insulated from the inner surface of the tube M bya refractory insulating material such "as aluminum oxide.

An oxide primary emission coating 41 is formed on the portion of sleeveM which can be heated by connecting a source of potential to the heaterleads at. This primary emissive coating 31 and the secondary emissivesurface '35 of the first dynode 2d face toward each other with thecontrol grid' l's mounted between them.

A secondary emissive coating 35 is formed on the central portion of theconcave, inwardlyfacing surface of each dynode.

A collector electrode or anode :28 is provided for intercepting thesecondary electrons from the fifth dynode 28. It is mounted to have aminimum of capacitance to ground so that even at high operatingfrequencies it will be free to float with respect to ground inaccordance with the product of the varying output current and anappropriate, purposely-included load impedance. To this end thecollector electrode 48 is supported in the manner shown in Fig. 1 sothat in all directions it is well spaced from any surrounding conductivestructure. For a similar reason the capacitance between the grid and thecathode as well as that between either of these elements and groundshould be kept at-a minimum,

The collector electrode 38 is supported in the center of openings 29, 22by the rod l9 and a glass bead 52 which are mounted within a hollowthreaded fixture 53 to form a vacuum-tight coaxial output receptacle'fil.

Respective leads for the cathode sleeve, the several dynodes and thegrid and the two heater leads 46 are separately sealed through the--envelope bottom E2 to form individual terminal pins 55. As shown, eachseal may consist of a metal tube silver soldered or welded over a holein bottom I2, and a glass bead 5'! fused over the end of the tube with alead sealed through it to form a terminal pin. Since the different leadsare brought through the bottom [2 at positions on both sides of assemblyl5 as well as beyond its ends, a number-of them do not appear in Fig. 1.However, a lead 58 for the final dynode 28 and a lead Gil for the seconddynode 32 do appear therein.

The principal parts of the envelope Ill-may be made of any one ofseveral metals which have appropriate mechanical properties (such asthat of holding a hard vacuum). However, one part, Thisis done, for"reasons which will be more fully set forth "below, to make use of thefact that silver has the property of diffusing oxygen, and that its rateof diffusion can be controlled by varyin the temperature of the silver.To provide a means for varying its temperature, the capsule 52 issurrounded by a heater winding 84 which may be connected to anadjustable source of potential 66. In this way oxygenmay be controllablyadmitted into the envelope from the surrounding air atmosphere at thesame time that other gases, such as nitrogen, are excluded.

Fig. 3 shows the condition of a preferred sec .ondary emissive coating35 according to the resent invention after it has been activated. Thepreferred coating includes two distinct strata. One of thestrata, whichmay be called the surface layer (or"interaction layer), 68, isindispensable. It absorbs the bombarding electrons and emits thesecondary electrons. The other, which may be called the backing layer,H3, is not essential. However, it is very useful as a reservoir ofmaterial into which the surface layer can progressively recede duringoperation (and from which its unexposed side can be replenished) as itsexposed side is sputtered away. The backing layer should be conductive,so that replacement electrons can flow through it from the dynode to thesurface layer to take the place of emitted secondary electrons.

The small squares, 69, shown in "Fig. 3, represent the free metal. Itwill be noted that in the preferred coating 35 there is more free metalin the backing layer'lil than in the interaction layer 63.

An important discovery of the present invention shows that the secondaryemission ratio (the ratio of secondary electrons emitted to primaryelectrons received) can be controlled, and maximized, by controllingtheratio of free metal to metal compound in the constituents of thesurface layer. Moreover, the secondary emission ratio can be maintainedat a high value, even when the coating is under extremely hard use,until it is entirely physically sputtered away, by continuouslymaintaining a proper ratio between "these constituents of the layer.

If there is either too little free metal or too much of it in thesurface layer, the secondary emission ratio will be adversely affected.On the other hand, the ratio of free metal to metal compound is notcritical in the backing layer except that there should be enough freemetal in this layer for it to be a fairly good conductor.

The coatings 35 may be formed on the dynodes in accordance with anysuitable prior art such as that of deposition by cataphoresis, or inaccordance with any suitable future art. Moreover, they may be made ofany satisfactory known secondary emissive material. Therefore, inaccordance with usual practice, each coating 35 will include, as-atleast one of its constituents,

a compound of a suitable metallic element and a suitable non-metal,usually a gas which may be either a gaseous element or a gaseouscompound. Most of the suitable metallic elements are either alkalineearth metals or alkali metals or a few other metals, such as magnesiumand beryllium, which are found in the first two groups of the periodicchart. However, not all of the metals found in the first two groups canbe considered suitable since 'a few of them, silver, mercury, gold andcopper have not yet been successfullycompounded to producegood secondaryemitters. Nor are all of the suitable metals to be found in the firsttwo groups. For example, thorium can be used for good secondary emittercoatings and the same promises to be true of aluminum.

. None of the suitable metals is a good secondary emitter in its purestate. To become useful as such, it must be combined with a suitablenonmetal. In general, a non-metal is usually suitable if it is normallygaseous at room temperature and if the suitable metal which is used hassuch an affinity for the non-metal in question that they readily combineto form a compound. Suitable non-metals comprise a group includinggaseous elements such as oxygen, hydrogen and the halogens (fluorine,chlorine and iodine) and a great variety of gaseous compounds such ascarbon dioxide, water vapor and carbon monoxide.

Apparently, it is generally true that, as initially formed, a secondaryemissive coating contains substantially no free metal, 1. e., itsfreemetal-to-metal-compound ratio is zero. This is because the suitablemetal constituents have a great afiinity for oxygen, and therefore anyfree metal which has not been converted into metal compound will becomecompletely oxidized as the coating is subjected to air. I havediscovered that it is because of this that some sort of activation hasbeen, and is, necessary before such a coating becomes a good secondaryemitter. In the past, activation has been accomplished by heating. Thiswas suggested by primary emitter experience and empirically it proved tobe useful.

As a result of my findings, it is now possible to understand whathappens in activation by heating and. therefore the reason why it isonly partially satisfactory.

It will be helpful to consider activation by heating before continuingwith the disclosure of the present invention. During heating the metalcompound forming the coating breaks down, for example, barium oxideseparates into free metal barium and oxygen. More oxygen is liberatedfrom the coating than metal vapor and therefore the free metal contentrises. Relatively little recombination takes place since much of theoxygen is absorbed by the getter and other parts of the tube. Asactivation proceeds, the increasing free metal component of the coatingbecomes rather homogeneously diffused throughout the coating because ofits high thermal energy. Therefore, the front-to-back conductivity ofthe coating is increased at the same time that the secondary emissionratio is improved. Up to this point, most of the results of heating aredesirable.

In the practice of this method, it was observed that the use of moreheat than necessary or the continuation of activation for a longerperiod than necessary did not deactivate the coating. As a result, itbecame an accepted practice to use activation by heating continuouslyduring operation. However, it is now apparent that this practiceshortens the actual life of the coating (as distinguished from itsuseful life). After activation by heating has caused the ratio of freemetal to metal compound to reach a certain value, more free metal vaporwill be liberated from the coating than oxygen. Therefore, excessiveactivation by heating, either as prolonged or extreme heatingpreparatory to operation or as continuous heating during operation,physically boils away the coating even though it does not deactivate it.In fact, this evaporation of free metal makes such heating entirelyprohibitive for certain particular coatings. A good example is a coatingwhich comprises magnesium. Though it affords a high secondary ratio whenproperly compounded, it is so volatile that if heated to a hightemperature or if continuously heated it will soon be entirelyvaporized. If the dynode which carries the coating comprises magnesiumthere will be suflicient vaporization to contaminate all of the interiorelements of the tube as well as to destroy the coating itself.

In addition to the fact that the use of continuous activation by heatingduring operation shortens the life of a secondary emitter, it also tendsto cause primary emission. Obviously, this is undesirable for asecondary emitter, since the secondary current must be controllable byelectron bombardment alone.

After initial activation by heating (this has usually been done attemperatures between 800 C. and 1000 C. for periods of from 10 minutesto 1 hour), it has been customary to use continuing activation byheating at about 500 C. all during operation. It should be noted that ifthis were not done, the useful life of the coating under high currentdensity conditions would be very short despite good initial activation.Continuous activation by heating is objectionable for additional reasonsbesides that of wasting away the coating(s).

Because of the necessity of maintaining the electrodes at such a hightemperature during operation, it is often impractical to pass a varyingsignal through a tube which is being worked close to its rated outputunless very fast-acting and critical temperature controls are used.Without such controls, a power tube which is already as hot as 500 0.might quickly burn up under certain signal conditions whereas underdifferent signal conditions the secondary emitter coatings of the finalstages might rapidly lose their efficiency. It would certainly be veryexpensive if not impossible to provide such controls. Moreover, most ofthe arrangements which have been used for providing dynode heating (forcontinuous activation) are actually incompatible with fast-acting,dynamically-controlled cooling. For example, in some arrangements, thedynodes have been thermally isolated from the rest of the dischargedevice so that heat provided by individual built-in heaters (similar tocathode heaters) would remain concentrated where needed.

It will be seen that continuing activation according to the presentinvention does not depend on maintaining a high temperature andtherefore permits continuous use of a fixed generous amount of coolingadequate to dissipate heat generated during peak points in the operationof the device. In other words, the tube may be run cool whereby it issafe to operate it at a very high and widely varying output power levelwithout danger of melting any of the dynodes and/or the anode.

It will be seen that the novel process for initial activation which isdisclosed below is also temperature-independent. For this reason, itwill be especially useful for activating tubes which are built withoutheaters and thermal isolation for their dynodes, but instead with heatconduction structures for cooling them.

Returning now to the disclosure of the present invention, the continuingtemperature-independent activation of the present invention isaccomplished, as has already been mentioned in a general way, byproviding in the tube gas molecules which will combine with free metalin the surface layer of a secondary emitter at a rate which iscomparable to that at which gas molecules are separatingoutfrom itsmetal-compound constituentis'). This con be done regardless of how thetube was activated in the first place. The gas which is provided ispreferably but not necessarily the same kind that is separating out.

The initial temperature-independent activation of the present inventionis accomplished by electron bombardment, preferably with the deviceunder hard vacuum.

After the device shown in Fig. 1 is assembled, it is pumped down to avacuum of about 10- mm. of mercury, and the tube dynodes are connectedto sources of direct potential to cause a current flow through thedevice. At first, since the dynode coatings will contain practically nofree metal, their secondary ratios will be very low. However, somezig-zag current will flow and there will be some electron bombardment ofeach dynode coating. This will cause a breakdown of the metal compoundin its surface layer. Free gas molecules will escape and some of it willbe absorbed by the getter and other elements of the tube.

Accordingly, a progression will be established in which the ratio offree metal to metal compound will increase; this will increase thesecondary current flow; and this will accelerate the rate of breakdownand hence the rate of activation. At the same time there is aprogressive improvement in the conductivity ofthe backing layer for thefollowing reasons: Once the secondary emission ratio of any dynode isgreater than one, a potential gradient will develop between the frontand back surfaces of its secondary emissive coating due to: (l) the highfront-to-back resistance v of'the coating, and (2) the loss of negativecharge from its surface layer. This will cause the ionic transport offree metal particles from the front toward the back of the coating.

This activationprocess should be terminated after a period of from twoto eight hours, when the rate of improvement has passed a maximum anddecreased to zero, i. e., has leveled off. This point can be recognizedby the fact that the anode current will have stopped increasing. By thistime the ratio of free metal to metal compound in the surface layer willhave attained the proper value for optimum secondary emission, and thebacking layer will contain quite a large amount of free metal and willaccordingly be quite conductive.

In the normal operation of the tube of Fig. 1, assuming that itssecondary emitters have been activated in some suitable manner, thetemperature of capsule 62 is adjusted to diffuse enough oxygen into theenvelope so that the recombination thereof with the free metalconstituent of the coating will offset the continuous breakdown of thecompound. Some recombination will occur because the thermal velocitiesof the oxygen molecules will cause them to come into contact with thesecondary emissive coatings. However, most of the recombination willoccur because ionized molecules will be attracted to the coatings. Manyof the oxygen molecules will be ionized in the zig-zag electron stream23 and will then fall back along it in the opposite direction ofmovement of the electrons' Each positivelycharged ion will strike themost negative dynode which it sees (usually next back toward the cathodefrom the place where the molecule became ionized). This ionicrecombining is very advantageous since it proceeds at the highest ratefor. the. dynode which is losing the greatest number of gasmolecules, 1. e., the dynode which is receiving the mostintensebombardment and. is providing the largest secondary current. The denseelectron currents moving to and from this dynode will produce more ionsin its vicinity than are being produced near any of the other dynodes.Consistent with this, I have found that in general if a proper amount ofgas is provided for the continuous restoration of any dynode, thisamount is substantially correct for all of them.

The zig-zag electron stream 23 is much narrower than the assembly [5 andonly extends across small central region of each dynode. Therefore, ionsformed in this stream are not significantly attracted by any part of thefield of the envelope (assuming that the envelope is at a lowerpotential than the dynodes) which extends into the inter-dynode space.Accordingly, most of the ions move in directions to impinge upon thedynodes and therefore most of them accomplish the useful functionexplained above. For this reason, the gas pressure within the envelopemay be as low as (and for certain gases lower than) 10- mm. of mercuryat all times.

Fig. 4 shows a modification which is suitable where thegas to beprovided within the envelope for continuous activation during operationcomprises hydrogen instead of oxygen. In this modification the silvercapsule (62) is replaced by a capsule 13, of a different material, e.g., palladium, which has the property of diffusing hydrogen. Inaddition, since hydrogen does not exist in substantial quantities in theatmosphere; a supply of this gas is confined about the outside ofcapsule 73 by a vessel E5. It should be noted that if the surface layer68 includes an oxide, such as barium oxide, at the start of operation,it may end up with that constituent converted to barium hydride.

In general, the residual pressure which should be left afterevacuation-pumping and the operating pressure which should be maintainedduring gas diffusion may be very nearly as low as a so-called hardvacuum of 10- mm. of mercury. The exact pressure will depend on theaffinity of the free metal font-he gas which is provided duringoperation. For example, fluorine is very active, so that if used at all,it should be used at lower pressure than that suitable for oxygen.However, fluorine would be harmful to a cathode of the type now in use,and therefore might not be preferred at the present time, this alsobeing true of chlorine.

An alternative or additional arrangement may be employed for providinggas molecules within the envelope. In it, the anode as will be formedof, or coated with, silver which contains a considerable amount ofoccluded oxygen. Though this sort of supply is exhaustible, it may lastfor a very considerable period of time. Moreover, inasmuch as the lifeof the secondary emitter coatings is not indefinite, but will eventuallyterminate when the coatings are finally physically sputtered away, it isnot necessary that the supply of oxygen be inexhaustible. In theoperation of the device, the amount of oxygen liberated from the anodewill vary directly with the bombardment of this electrode. Therefore,the supply of gas will automatically change in the proper direction tocause the rate of reconstitution of the coatings to follow changes intheir rate of breakdown. By providing adjustable anode cooling, as byblower 3! of Fig. l, or adjustable anode heating, as by a built-inheater winding (not shown) it will be possible to control the rate atwhich the anode diffuses the occluded oxygen by controllably influencingits average temperature. If desired, a tube element especially intendedfor the purpose, rather than the anode, may be used for containing theoccluded oxygen. Moreover, wherever it is stored, the supply of oxygenneed not necessarily be in the form of occluded free gas. For example,it may be bound in a compound and releasable therefrom as compound isdecomposed by being heated.

Once the tube is in operation, thermal diffusion of free metal in thebacking layer 10 will be relatively slow due to the low operatingtemperature. This fact, in combination with the continuous ionictransport of free metal from the front toward the back of this layerwill tend to establish and maintain a reserve of nearly-pure,highlyconductive metal deep in the coating.

In testing one embodiment of the present invention in which thesecondary emissive coatings included a mixture of magnesium oxide andbarium oxide, it has been found that by continuously providing a properamount of oxygen within the envelope it was practical to maintain anaverage secondary emission ratio of five for a period of one hundredhours with the final dynode emitting a current of higher than one ampereper centimeter squared. Moreover, since it was possible to operate thetube cold, no difliculty was occasioned due to evaporation of magnesium.

The non-metal constituent (of the compoundingredient of the secondaryemissive coatings), which is described herein as being gaseous at roomtemperature and as having an afiinity for the metal constituent,includes all of the substances which are referred to as oxidizing agentsin both the restricted and broad meanings of the expression. In therestricted meaning, these include the element oxygen and a number ofcompounds including oxygen, such as water vapor, carbon monoxide andcarbon dioxide, which can provide an oxygen molecule to join with amolecule of the metal. In the broader meaning they further include anumber of other substances any one of which has such a tendency to joinwith the metal that it can provide a molecule to displace an oxygenmolecule already joined to the metal.

It is apparent from the foregoing that the present invention provides animproved discharge device, as well as a process for making such a deviceand a method for using it, in which the secondary emitter coatings canbe depended upon to have long, useful lives even under high currentdensity conditions.

What is claimed is:

1. A discharge device comprising a vacuum envelope containing a dynodehaving a secondary emissive coating including a, compound of a metalwith a non-metal, and means for continuously reconstituting said coatingby providing within the envelope in gaseous form a supply of a nonmetal,such as said first-mentioned non-metal, for which said metal has anaflinity. said means comprising a portion of said vacuum envelope whichis formed of a material which has the property of selectively diffusingsaid last-ment-ioned non-metal.

2. A discharge device as in claim 1 in which said non-metal for whichsaid metal has an affinity is oxygen, and said material forming aportion of the envelope is silver.

3. A discharge device as in claim 1 in which said means comprises meansfor controllably heating said portion of said envelope.

4. A discharge device as in claim 1, in which said non-metal for whichsaid metal has an affinity is hydrogen, and said material forming aportion of the envelope is palladium.

5. A discharge device as in claim 1, in which said means includes areservoir connected to said envelope around said portion and containinga supply of said non-metal for diifusion through said portion into saidenvelope.

DIETRICH A. JENNY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,566,279 King Dec. 22, 19252,228,945 Bruining et a1 Jan. 14, 1941 2,242,644 DeBoer May 20, 19412,393,803 Nelson Jan. 29, 1946 2,497,911 Reilly et al Feb. 21, 1950

